The Phone Rings at Midnight
It’s midnight on a Wednesday, and as a SOC analyst, you’re just settling in when your phone buzzes with a dreaded alert:
"BytesDownloaded exceeding threshold."
An unusual activity alert from CloudWatch Metrics signals that something’s not right. Multiple S3 buckets in your AWS environment are involved. Before you can catch your breath, a flood of S3 events start pouring in.
Your heart sinks as you realize this is far from normal..
But what could be happening?
Before diving deeper, let’s set the stage.
Amazon S3 is one of the most widely used storage services in AWS. Enterprises trust it to store critical data that powers their business applications. For many organizations, this data isn’t just important—it’s the heart of the business. Any compromise could have fatal consequences for the company's health.
Because of the value and sensitivity of the data stored in S3, it becomes a prime target for attackers. They know that if they can get their hands on your S3 data, they can do significant damage, whether it’s through data theft, ransomware, or other malicious activities.
The security of this data is entirely dependent on how well your S3 buckets are configured and protected.
Guarding the Gates: Key Security Features of S3
There are multiple ways to control access to an S3 bucket and safeguard the data stored within it. Let's break them down:
1. Block Public Access
By default, S3 buckets block public access, but this can be overridden. It’s essential to ensure public access is explicitly blocked unless there’s a very specific reason to allow it. Misconfigurations here are a common cause of data exposure.
2. Bucket Policies
Bucket policies are resource-level policies that help ensure access to your S3 buckets is restricted to authorized users only. By setting granular policies, you can control who has access and what actions they can take.
3. IAM Policies
It’s crucial to ensure that IAM policies controlling user permissions are set to the least privilege principle. This prevents users from having more access than they need, reducing the risk of exploitation.
4. Encryption
Encrypting your data at rest is another critical layer of defense. AWS provides several options for S3 encryption:
Server-side encryption using AWS-managed keys.
Customer-managed keys with AWS KMS.
Client-side encryption, where data is encrypted before it’s uploaded.
Access Control and Encryption Are Vital, But They’re Not Enough
While access control and encryption can safeguard your data, they won’t tell you when something’s gone wrong. That’s where monitoring comes in. To detect suspicious activities or potential breaches, you need to have CloudTrail and CloudWatch in place.
Detecting Suspicious Activity: CloudTrail and CloudWatch
CloudTrail: Your Audit Trail in AWS
AWS CloudTrail logs all the API calls in your environment. Whether someone is listing objects in an S3 bucket or downloading large amounts of data, it’s all captured here. In this case, the S3
events that came flooding in? CloudTrail recorded them, providing crucial details about who accessed what and when.
By analyzing the CloudTrail logs, you can trace the suspicious activity back to its source—whether it's a legitimate user acting outside normal patterns or a compromised account.
CloudWatch: Real-Time Monitoring
CloudWatch Metrics allows you to monitor your environment in real-time. In our midnight scenario, it’s CloudWatch that triggered the alert for BytesDownloaded exceeding a threshold, signaling that someone was potentially exfiltrating data from the S3 buckets.
These two tools work together to give you visibility into what’s happening in your environment, helping you catch threats before they escalate.
So, What’s the Real Culprit Here?
You’ve ruled out the typical suspects:
Public access to the bucket is blocked.
The IAM policies are tight, with no excessive permissions granted.
So, what else could it be?
Could it be a compromised access key?
The Dangers of Compromised Access Keys
Access keys are like digital keys to your AWS kingdom. If they fall into the wrong hands, attackers can blend in, using legitimate credentials to perform malicious actions—such as exfiltrating sensitive data from your S3 buckets.
These keys can be compromised in various ways:
Phishing attacks targeting employees with access.
Exposed keys in code repositories.
Insecure logs containing sensitive credentials.
Once attackers have these keys, they can operate quietly under the radar, leveraging legitimate access to your environment.
Investigating the Attack: CloudTrail Holds the Clues
With a compromised access key suspected, you dig deeper into the CloudTrail logs.
As you sift through the logs, a pattern begins to emerge. A legitimate user account is being used—but the access patterns don’t match up. The requests are coming from an unfamiliar IP address at an unusual time. The ListBucket and GetObject calls show someone quietly retrieving data, likely using the stolen key.
You’ve found the smoking gun: a compromised access key.
Taking Action: What to Do Next
Now that you’ve identified the cause, immediate action is crucial. Here’s what you should do:
Revoke the compromised access keys immediately to stop the attack in its tracks.
Rotate credentials for all affected users and ensure no further unauthorized access can occur.
Review IAM policies to ensure they follow the least privilege principle.
Enable MFA (Multi-Factor Authentication) for all sensitive accounts to add an extra layer of security.
Audit CloudTrail and CloudWatch regularly to ensure no suspicious activity goes unnoticed.
Encrypt your data using AWS KMS and enable versioning to safeguard against future incidents.
Lessons Learned
This attack wasn’t due to a publicly accessible bucket or an overly permissive IAM role. Instead, it was the result of a compromised access key, a more subtle and dangerous form of attack.
The key takeaway here? Prevention is essential, but so is detection. Without real-time monitoring and audit logging, you may never spot an attack until it’s too late.
The next time your phone rings at midnight, will you be ready to detect and respond before your data is compromised?